Video transcript

- [Voiceover] So if you
are like most of us, your body probably sweats when it is warm, when your environment is warm, and you probably realize that it sweats in order to cool itself, in order to keep the
body from overheating. But you probably have wondered, "Well, how does this work? "What is actually causing that?" And the simple answer is it's happening-- or what's allowing your body to cool-- the chemical process,
I guess you could say the physical process is happening is evaporative cooling. Evaporative cooling. Which is really the
notion that as that water, those beats of sweat vaporize, it's actually gonna cool your arm down. But that begs the question, how does that actually happen? And so let's just
visualize it a little bit. Let's just say that is your arm, so this is your arm right
over here, so let's... I don't draw my best... Draw a quick arm right over here, so okay, that's your arm,
and it's got beads of sweat. Let's say it's a hot environment it's got beads of sweat right over here. And if we were to zoom in on that sweat, if we were to zoom in on that sweat, we would see the
constituent water molecules and sweat is mainly H20. It is mainly water. Now when we talk about the
temperature of something, we're talking about the
average kinetic energy. Each of the individual molecules, they all have different kinetic energies. They're all bouncing
around in different ways and transferring the momentum
in all different ways. And so you can imagine a reality. Maybe this one has a
fairly high kinetic energy. It's moving in that direction. This one has a lower kinetic energy. moving in this direction. Maybe this one has a
medium kinetic energy, moving in this direction. Maybe this one has a
really high kinetic energy moving in that direction. And so we've already talked about how hydrogen bonds in water between the partially negative end and the partially positive ends. That's what keeps the water together as these things move past
and flow past each other. What gives the water its cohesion is these hydrogen bonds. But if all of a sudden-- remember, we're talking about
the average kinetic energy-- but even if we're at room temperature, and the average kinetic
energy isn't so hot, you might have individual particles, individual molecules that actually have quite a high kinetic energy and if they're in the right place, if they're near the surface
and their kinetic energy is high enough to break the hydrogen bonds with neighboring water molecules, and to overcome the
pressure in the atmosphere, so let's say that this is,
these are just gas molecules in the atmosphere here. But it's enough to break
free and none of these things bounce into it and force it back to form hydrogen bonds. This thing could actually break free and enter and become water vapor. And become in its gaseous state. And it'll be so far apart
from other water molecules that it won't form hydrogen bonds anymore. So by vaporizing or by this
process of evaporation, what's happening? Well if your highest
kinetic energy particles or some of your highest
kinetic energy particles are able to escape, what's going to happen to the average kinetic energy? Well as the highest kinetic
energy things escape and those are the ones that
are most likely to escape, well then your average kinetic
energy is going to go down. So average kinetic energy is going to go down. Or another way of saying it, is that your temperature
is going to go down. Your temperature is going to go down because as these molecules
turn into water vapor, they're going to be the
highest kinetic energy, energy is transferred to
them, and then they escape. And so what's left over is going to have a lower average kinetic energy. And you're saying, "Well, how does that "actually cool down my hand?" Well, your hand is made
up of molecules as well. So let's say this is the
surface of your hand, those are the molecules, they have some average kinetic energy, they are kind of vibrating in place, especially if we're
talking about they're... they're a solid. And so maybe I'll draw the more, you know, they're vibrating like this, they're bonded to each other in some way. I won't go into the details of what types of molecules these are, but then if you have your
water molecules here, water molecules that are
sitting on the surface, and I'm drawing this is
kind of a cross-section. Let me draw the water molecules. I'll draw them as blue molecules. So this is an H2O right over here. H2O. This is an H2O. And this is an H2O. And they have some hydrogen bonding, so there is some hydrogen
bonding going on. Well, as the high kinetic
energy water molecules escape, I'll say this one right over here escapes, and so the average kinetic energy of what's left over is lower, so then the temperature has gone down, and now your body molecules, the ones that are all warmed up, and because of whatever's
going on inside of your body, well, those can now bump into, they can vibrate and bump
into these water molecules and increase their kinetic energy more than the ones that have
the most kinetic energy. Those might escape again. And so it's a--one way
of thinking about it is that all that heat is being used to allow these individual water molecules to escape in order to vaporize. And so that heat is leaving your body, so it allows you to cool down. Cooling down happens by
heat actually leaving. So that's how evaporative... I wrote evaporative cool... That's how evaporative cooling... That's how evaporative
cooling actually works.